Siemens S5-115U CPU 941/942 PG Connection Troubleshooting Guide

The SIMATIC S5-115U programmable logic controller remains in service across legacy installations worldwide. When a CPU 941 or CPU 942 stops responding to the programmer (PG) port, technicians must isolate the fault across three distinct subsystems: the CPU module, the EPROM submodule, and the serial interface adapter chain (TTY current loop to RS-232 conversion). This guide consolidates field-proven diagnostic procedures, voltage thresholds, and replacement strategies for the S5-115U PG link, drawing on the SIMATIC S5-115U hardware manual and the S5-115U Programmer's Manual.

Refer to the Siemens Industry Online Support portal for the latest S5 legacy documentation set. The SIMATIC S5-115U Automation System Manual (6ES5998-...") and the S5-115U Programmer's Guide (PG 615/635/685/710) remain the canonical references for CPU 941/942 commissioning and online operation.

1. Problem Definition: "No Connection with PG"

The fault signature is unambiguous: STEP 5 returns Error 1211: Online: No connection to PLC or the equivalent CPU does not respond dialog when initiating an online function (status, force, download, upload, or block compare). On the S5-115U CPU front panel, the RUN indicator shows green (or STOP if the program halts), no red LED is lit, and the battery warning LED may or may not be active.

Three failure modes are commonly confused at the cabinet:

2. SIMATIC S5-115U PG Interface Architecture

The S5-115U uses a 15-pin sub-D female socket on the CPU faceplate labelled PG/PC (or simply PG). The electrical standard is TTY 20 mA current loop — not RS-232, not RS-422, not RS-485. This is the single most common reason technicians fail to establish a link: a passive 9-to-15 pin cable does not perform the required current-loop / voltage-level translation.

2.1 Pin Assignment of the S5-115U PG DB15 Socket

Verify the exact pin-out for your specific CPU variant in the Siemens S5-115U hardware manual; the CPU 941, 942, 943, and 944 share the same DB15 footprint, but signalling levels for some sub-models differ when the 6ES5 491-... is substituted for a 6ES5 942-...

2.2 Why a Plain RS-232 Cable Fails

RS-232 is a voltage-mode standard using ±3 to ±15 V levels referenced to a common ground. TTY 20 mA is a current-mode standard where logical 1 corresponds to current flow and logical 0 to current interruption across an isolated pair. Connecting an RS-232 line driver directly to a TTY port will:

• Produce no current source — the CPU's opto-isolated receiver will see a high-impedance input and stay in one state.
• Risk damage to the line receiver IC on the CPU if the RS-232 voltage exceeds the TTY receiver's breakdown rating.
• Fail to establish the half-duplex handshake expected by the S5 communications stack.

An active adapter is required: it must source the 20 mA current loop on at least one pair and convert to RS-232 levels (or USB in modern PGs). Passive wiring harnesses such as the 6ES5 734-... or 6ES5 736-... used between S5 PGs and S5 CPUs already contain the active current-source electronics, but a third-party PC card must emulate that function.

3. Required Tools and Adapters

For a competent S5-115U PG link investigation, the technician should have the following on the bench before opening the cabinet:

4. Step-by-Step Diagnostic Procedure

Follow this decision tree in sequence. Do not skip to the final step (board repair) without isolating the cable, the adapter, the EPROM, and the connector first.

Step 1 — Confirm the Cable and Adapter

1. Disconnect the suspect cable from the CPU 941/942.
2. Connect the cable to a known-good second S5-115U CPU (or any S5 PG port known to work). Attempt an online function from STEP 5. If this link is successful, the cable and adapter are exonerated.
3. If the link still fails on the second CPU, suspect the cable / adapter, not the target CPU. Swap the adapter and repeat.
4. Perform a TTY loopback test: short pins 2–4 and 3–5 on the DB15 end of the cable. A working adapter with self-loopback should echo PG keystrokes to the screen in terminal-emulation mode.

Step 2 — Verify the DB15 Socket Voltages

With the CPU powered, the adapter connected, and STEP 5 attempting an online connection, measure the following on the DB15 socket of the suspect CPU:

If a "bad voltage" is found on the DB15 connector (as in the field report of the CPU 941), the most common cause is one of the following:

• Cold solder joint at the DB15 through-hole pins on the CPU PCB — reflow with a 60/40 or lead-free solder station at 340–370 °C.
• Lifted pad on the PCB trace from the DB15 to the interface IC — repair with a jumper wire of 30 AWG Kynar wrap-wire, secured with UV-curable solder mask.
• Cracked connector body allowing intermittent contact — replace the DB15 socket with a crimp or PCB-mount equivalent (AMP 747275-2 or equivalent).
• Corroded pin due to humidity / H₂S exposure — clean with isopropyl alcohol and a fiberglass brush; replace if the pin is pitted.

Step 3 — Check the EPROM Submodule

The S5-115U CPU 941/942 stores its user program in an EPROM submodule (or EEPROM on later revisions) that is read into battery-backed RAM at startup. The procedure for transferring the program from a faulty 941 to a spare 942 is:

1. Power down the source CPU (941). Note the orientation of the EPROM submodule (notched end / pin 1 indicator).
2. Remove the EPROM submodule from the 941 by carefully lifting the locking lever or unscrewing the retainer (varies by submodule family — 6ES5 375-... vs 6ES5 376-... vs 6ES5 378-...).
3. Visually inspect the EPROM pins for oxidation. Clean with a soft eraser or contact cleaner if needed.
4. Install the EPROM into the spare 942, observing correct orientation.
5. Power up the 942. Observe the front-panel LEDs: RUN should come on green within 3 – 5 seconds. If RUN does not come on, the EPROM may be incompatible (wrong submodule family for the 942 firmware) or corrupted.
6. Press the ERASE button on the CPU faceplate if fitted, to clear RAM and force a fresh reload from EPROM. This is a known requirement on some 942 firmware versions before the PG link will establish.

Step 4 — Upload via Serial Instead of EPROM Transfer

As an alternative to EPROM transfer, you can upload the program from the source 941 to the 942 over the serial link:

1. Connect the working PG to the 941 over its (known-good) serial link.
2. In STEP 5, select File > Upload to PG (German: Datei > Bausteine laden > Übertragen PG).
3. Save the project to disk on the PG.
4. Connect the PG to the 942 and select File > Download to PLC (Datei > Bausteine laden > Übertragen AG).
5. Force a STOP → RUN transition on the 942 to start execution.

This path requires the source 941 PG port to be functional, which is not the case in the reported scenario. EPROM transfer is therefore the preferred fallback.

Step 5 — Board-Level Repair (Last Resort)

If Steps 1–4 have not restored PG communication, the fault lies on the CPU PCB. Two sub-modes are likely:

• DB15 connector fault: identified by abnormal DC measurements on pins 2/3/4/5. Repair by reflowing or replacing the connector as described in Step 2.
• Serial-interface IC failure: typically a Siemens 6ES5 942-... ASIC or a commercial UART (e.g. SAB 8251 or Zilog 8530) feeding the TTY line driver. Replace the IC if competent in SMD rework; otherwise swap the CPU module.

When the user reported the symptom and the assistant confirmed the DB15 connector was at fault, the field resolution was a board-level repair. Reflowing the connector or replacing the DB15 socket restored the link.

5. Battery Status and Its Effect on PG Communication

The CPU 942 reports battery status via a front-panel LED (typically red, labelled BAU or BAT). A weak or missing backup battery does not, by itself, prevent PG communication — the program will still load from EPROM, the scan will still run, and the serial port will still respond. A low battery does mean:

• Retentive flags, timers, counters, and process-image values will be lost on power-down.
• Some firmware versions enter a safe-state that holds the CPU in STOP and refuses PG online operations; this is rare but documented.
• The battery LED on the front panel will be lit, which can be confused with a more serious fault.

Replace the lithium backup cell with a 3.6 V, ≥ 1.0 Ah primary lithium thionyl chloride cell (e.g. Tadiran TL-5186, Saft LS-14250, or a Siemens-rebranded equivalent). Observe polarity: the + terminal is keyed on the holder. The old battery must be disposed of as hazardous waste per local regulation.

6. Common Error Messages and Their Meaning

7. Verification Procedure After Repair

After completing the repair — whether it is a DB15 reflow, EPROM transfer, or board swap — perform the following verification sequence to confirm the PG link is fully restored:

1. From STEP 5, initiate an Online > Connect to AG and confirm the dialog box populates with the CPU type and firmware version.
2. Issue a Status (Status var) request on a known flag (e.g. M 0.0) and verify the response reflects process state.
3. Run a Block compare between the PG project and the CPU content. Mismatches indicate EPROM or RAM corruption, not a link issue.
4. Force a VAR write to a non-process-critical bit and confirm the change persists through a scan cycle.
5. Cycle power and confirm the link re-establishes automatically (some adapters need re-handshake; some do not).
6. Replace the cabinet cover, restore IP rating if applicable, and document the fault in the maintenance log with the failed component and serial number.

8. Spare-Part Strategy for Installed-Base S5-115U

Given the S5 family is in extended / legacy support with Siemens, many components are no longer manufactured. The recommended spare-part holding for a critical S5-115U site is:

9. Preventive Maintenance Recommendations

• Quarterly: cycle the CPU from RUN to STOP to RUN and verify a successful online connection from the engineering workstation.
• Annually: measure the backup-battery voltage under load (a 3.6 V cell should still read ≥ 3.4 V at 1 mA load). Replace at < 3.2 V.
• Every 3 years: clean the DB15 connector with isopropyl alcohol and inspect the cable strain relief.
• Every 5 years: replace the backup battery regardless of measured voltage (typical service life is 5–10 years depending on cell).
• After any cabinet HVAC or filter change: re-verify PG link, as dust ingress is a leading cause of intermittent serial faults.

10. Field-Proven Caveats and Edge Cases

• Some USB-to-TTY adapters (e.g. cheap FTDI-based units) do not source the 20 mA loop and will fail to handshake with the S5-115U even though they enumerate correctly on the PC. Use only adapters explicitly listed for S5 (e.g. IBH USB-S5-Adapter, ACCON-COM-Cable USB).
• Mixing EPROM submodule families between CPU generations can corrupt the boot sequence on the target CPU. The CPU 941 accepts 6ES5 375-0LA... and 6ES5 378-...; the CPU 942 adds support for 6ES5 376-...; the CPU 943/944 extend further. Verify compatibility with the Siemens S5 memory submodule compatibility matrix.
• A weak backup battery on the source 941 may have left EPROM-resident data intact but corrupted RAM-resident volatile data, producing a confusing fault picture on power-up. Replace the battery before pulling the EPROM.
• The DB15 socket on the CPU faceplate is a frequent failure point because it is the mechanical interface most exposed to vibration and operator wear. In environments with high vibration (compressors, stamping presses), consider a strain-relieved cable and a periodic retorque of the DB15 locking screws.
• STEP 5 V6.6 is the last generally available release; for Windows 10/11 compatibility, run STEP 5 in a Windows XP SP3 virtual machine with a USB-passthrough for the active adapter. Some third-party adapters ship 64-bit signed drivers; verify before deploying.
• If a CPU 942 originally had a working PG link and loses it after a firmware update or submodule change, revert the submodule first. A rare firmware bug in some 942 firmware revisions disables the serial port until the next power cycle.

11. Frequently Asked Questions